How to mock a class member object method which got called from an inner class

I want to write a test for class A which has an inner class C and private member class B. A calls class C and C calls one private method of A to execute 'make' behaviour B. Below code will give you the clarity.

I want to mock B and mock behaviour B.make. Please suggest a solution. I am writing a test for class A

public class A {
    private ArkSolver B = new ArkSolver();

    public Itinerary execute() {
        other1();
        other2();
        buildArkTripPlans();
        return other3();
    }

    private void private1OfA() {

        ArkTripPlan arkTripPlan = new C.LeafArkTripPlanner(ark).plan();
    }

    private ArkTripPlan private2ofA (Ark ark) throws VRPSolverException {
        ArkTripPlan tripPlan = B.make(arg1,arg2,arg3);

        return tripPlan;
    }

    private class C {
        private Ark leafArk;

        public ArkTripPlan plan() throws VRPSolverException {
            return createTripPlan(leafArk);

        }
    }
}